There's a maddening amount of posts here, but it is the best place online to get information on the ST-1 and the S3 buggy. R.Saxton and many other TTR factory drivers post here. I personally don't have the truck, but I do run the S3. As for your concerns...
1) The DX-3 will work with either set of servos. I don't know the stats for the steering servo that comes with the truck, but Id bet the JR Z590 ( which is the better of the two servos that come with the DX-3) would also be better than the stock one in the truck. For break in or light bashing, it will be fine, but for racing...definately get a servo with 120 Oz/In or more torque. Among the most popular are the Airtronics 94357 or 94358, Futaba S9451, Hitec Titanium series, or JR Z series. Once you do this, put the Z590 on throttle and your'e set.
2) I believe that a flat style Rx pack is what is needed on the ST-1. The buggy uses flat so Id guess that the truck is the same. The box is plenty large enough to fit both Rx and the battery. The S3 box is smaller than the truck, and I have no problem with mine.
The above is a decent and inexpensive charger for the Rx battery. Whatever charger you use, just be sure it can charge NiMh cells and don't charge the pack at more than 1 amp.
3) Being a racer, I prefer to use 30%. I honestly don't think there will be any real difference as far as tuning issues. If you have problems with 30, you'll have problems with 20, or any other level. The main thing is going to be youre break in procedure. If you have one, use a heat gun on the engine block and use a temp gauge to aim at the glow plug. Get the temp up to 180-210...then start it up. Smoothly roll on and off throttle up to aprox half trigger. Drive it around slowly like this for 3-4 minutes.Don't blip or jab on and off throttle...roll smoothly. Also do not get W.O.T ( wide open throttle). Once the 3-4 minutes is up, kill the engine. You can do this by pinching the fuel line near the carb inlet or using your foot ( with shoe on of course ).Hold the chassis upside down, put a toe on the flywheel through the hole on the bottom of the chassis. Or you can use a rag and plug the stinger on the exhaust. Everyone has a preferred method and there's pros/cons for all three. I use the toe on the flywheel, what ever is easiest for you will be fine. Once the engine is stopped, you should bring the piston to B.D.C. If you don't the sleeve will cool around the piston, not allowing it to cool to it's proper size and this will shorten the life of the engine. You can "feel" when it is at B.D.C. When you feel no resistance, it is there. Or, before the engine is ever started, you can take the glow plug out, look down in the engine, roll the flywheel by hand and see when it is at B.D.C. Now, flip the buggy over, and you can mark the piston. A lot will say to use a Sharpie, but in my experience, this wears off. I use a body reamer to make a small indentation on the flywheel. Now when I kill my engine, I simply look for the mark on the flywheel and I know the piston is at B.D.C. Continuing break in, let the engine cool for 10-15 minutes. Do this procedure...start it up, run it slowly, smooth throttle input, run it for 3-4 minutes for 6-8 times, kill the engine, bring piston to B.D.C and let it cool...this should be equal to approximately a half gallon of fuel. This process is called heat cycling. After the heat cycle period, you can start to run full tanks. You can start to push it to W.O.T, but don't hold it at W.O.T for more than 3 seconds. Run it for another half gal. or so and you should be fine to start race tuning from there. Fuel recomendations...Byron's, O'Donells, TQ, Blue Thunder, and Werks all have quality products. The main thing, once use you one brand, try to stick to that brand at all times. Same with nitro %. It won't hurt anything , but you will have to tune and retune everytime you change either brand or %.
4) You can use the pull start...just be sure to remember it is not a lawn mower....short ( 5-6" ) pulls are all that is needed. After a while though, you may tire of getting blisters from the chord and from bumping your hand on the exhaust as you're restarting a hot engine. When you do tire of this, you can try using a starter box.
5) Cleaning...you can use the search engine here and try to find a thread. There's numerous ones posted already. If you at the track, and it is dry out, most people simply use ( if it is provided by the track, most tracks do) an air compressor to blow dirt off in between heats. If it is wet, or at the end of the day... remove the radio tray, cover the engine head and carb with a plastic baggy, use Simple Green or denatured alcohal and spray the truck down, let it sit for a few minutes and then use the air hose to blow it all off. Then grab some WD-40, spray liberally and then blow off the excess. This is just a basic idea..to get you started.
Sorry for the short story, lol...anyway, as far as thinking of other things....I could write volumes...it would likely be better if you post here any other questions you may have. Good luck and ROCK ON TTR